Energy tester for testing blasting machines that detonate electro-explosive devices

ABSTRACT

An apparatus for testing the energy output of a blasting machine used for detonating electro-explosive devices to determine if the blasting machine will supply sufficient energy to detonate the electro-explosive devices. The tester is capable of determining within plus or minus one Joule the output of a 25 Joule blasting machine. The energy tester includes go and no-go indicator lights to inform the operator of the blasting machine whether or not he has sufficient energy.

United States Patent [1 1 1111 3,865,028

Thakore Feb. 11, 1975 [541 ENERGY TESTER FOR TESTING 2,278,113 3/1942 worm-.. 324/151 2,398,606 4/1946 ang 324/106 BLASTING MACHINES THAT DETONATE 3,704,393 ll/l972 Digney, Jr. et a1 102/702 R ELECTRO-EXPLOSIVE DEVICES Kaushik H. Thakore, Sidney, NY.

The Bendix Corporation, Southfield, Mich.

Dec. 26, 1972 Inventor:

Assignee:

Filed:

Appl. No.1

References Cited UNITED STATES PATENTS 12/1914 Hiatt 324/106 Primary ExaminerAlfred E. Smith Assistant Examiner-Ernest F. Karlsen Attorney, Agent, or Firm-Raymond J. Eifler [57] ABSTRACT An apparatus for testing the energy output of a blasting machine used for detonating electro-explosive devices to determine if the blasting machine will supply sufficient energy to detonate the electro-explosive devices. The tester is capable of determining within plus or minus one Joule the output of a 25 Joule blasting machine. The energy tester includes go and no-go indicator lights to inform the operator of the blasting machine whether or not he has sufficient energy.

16 Claims, 4 Drawing Figures fifiFEfiE/VCE SIG/VAL GEM A c m/uee Mq/vosmae A g 5 ,4, c/Rcy/T MUlT/V/BRA G0 0 amsw/va Q g c MAC/l/A/E' COMPARATOR ;1 4,0 0 s/a/mL A 6 D 0 R awe/@4701? 5 ENERGY TESTER FOR TESTING BLASTING MACHINES THAT DETONATE ELECTRO-EXPLOSIVE DEVICES BACKGROUND OF THE INVENTION This invention relates to blasting machines for detonating blasting caps, electro-explosive devices, or the like. The invention is more particularly related to capacitor discharge type blasting machines and an apparatus for determining when they are operating properly.

Basically, blasting devices for firing explosive devices include a source of power such as a battery, an oscillator, a transformer responsive to the oscillator for stepping up the pulses therefrom, a storage capacitor which is charged by the pulses from the transformer, and a trigger circuit which allows the energy stored in the capacitor to discharge and fire an explosive device. The energy stored in the capacitor is discharged through the explosive device by means of a triggering circuit which may be operated automatically or manually. An example of a capacitor discharge type blasting machine of the aforementioned type, with which this invention may be used, may be found in U.S. Pat. No. 3,704,393 entitled Capacitive Discharge Type Blasting Machine to F. J. Digney et al, issued Nov. 28, 1972.

The use of portable battery type blasting machines to initiate large blasting rounds, in preference to direct use of commercial power lines, as a source of electrical current, is well recognized, especially for underground blasting operations in tunnels and mines that employ delay electric blasting caps. Employment of capacitive discharge type blasting machines to initiate electric blasting caps, including both instantaneous and delay electric blasting caps, is especially well known, and some capacitive discharge blasting machines have been provided that initiate relatively large numbers of electrical blasting caps in a single blasting round. The number of electric blasting caps that can be initiated in a single round by a given blasting machine depends in part on the way in which the electric blasting caps are connected to the blasting machine. It is well known in the blasting art that numbers of electric caps can be initiated from a given power source in a single blast by arranging the caps in series or in parallel. The relative simplicity of making parallel firing circuit hook-ups with many caps in the round have created demands for dependable means of firing all the electric blasting caps that are connected in the firing circuit.

In certain blasting operations such as those performed in tunnels and shaft mining, it is generally desirable to connect as many as 150 blasting caps together in a parallel circuit, principally because such a circuit permits rapid connection of the blasting caps with minimal possibility of error. However, to insure that all the blasting caps are fired, the blasting machine must always deliver a given minimum energy each time it is fired; otherwise, all of the blasting caps may notbe fired. Experience has shown that blasting machines occasionally do not provide all the power they are supposed to, thereby failing to detonate all the blasting caps they are supposed to.

SUMMARY OF THE INVENTION This invention provides an apparatus for determining that the energy discharged from the output of a blasting machine is above a predetermined value and therefore the blaster discharges sufficient energy to detonate a predetermined number of electro-explosive devices.

The invention is an apparatus for testing the rated energy of a blasting machine characterized by the fact that it is connectable to receive the output energy from a blasting machine (1) and transmit the energy to a translator (2) to provide a signal, which is related to a predetermined acceptable energy level. The comparator (6) then provides indication (7) as to whether or not the energy discharged from the blasting machine (1) into the tester is above a predetermined acceptable amount. The tester circuitry is further characterized by the fact that it is capable of simulating a load of series connected or parallel connected blasting caps and that the output of the reference signal generator (3) is a pulse (52) having a predetermined magnitude and duration, that is compared by the comparator (6) to the output of the signal generator (2), which is a non-pulse type signal, the magnitude of which varies with respect to time.

In one embodiment of the invention, the tester, used in combination with a blasting machine (I) of the type that discharges energy stored in a capacitor to initiate at least one electro-explosive device, comprises: a first circuit (201214) for receiving the energy discharged from said blasting machine and producing a first signal (B), the magnitude of which varies with respect to time, and which is related to the amount of energy discharged by said blasting machine; a second circuit (401-413, 501-514) for generating a reference signal (A) which is related to a predetermined amount of energy, the circuit for generating a reference signal including a trigger circuit (401-413), responsive to the receipt of the energy received from the blasting machine (1), for generating a trigger pulse upon receipt of the blasting machine output energy, and a monostable multivibrator (501-514) for producing an output pulse signal (A) in response to the trigger circuit pulse; a third circuit (601-609) for comparing the reference signal (A) to said first signal (B) and generating a third signal (C) when the magnitude of the first signal (B) exceeds the magnitude of the reference signal (A) during the presence (T2) of said reference signal (A); and a fourth circuit (7l0-729) responsive to said third signal (C) for indicating the presence of the third signal (C) whereby the indicating circuit gives an indication (GO) that the output energy received from said blasting machine is above a predetermined amount and therefore the blasting may be employed to detonate its rated number of blasting caps.

Accordingly, it is an object of this invention to provide an apparatus for determining whether or not the energy produced by the output of a blasting machine is at or above an acceptable level.

It is another object of this invention to prevent the application of energy to a plurality of electrically connected electro-explosive devices or the like when there is insufficient energy available to detonate all the devices.

It is still another object of this invention to provide the operator of a blasting machine with a device that allows him to test, rather easily, the amounts of energy available in his blasting machine.

It is a further object of this invention to provide a device which senses the total output energy of a blasting machine rather than the voltage level at the output of a blasting machine.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a system incorporating the principles of this invention for evaluating the operational capabilities of a blasting machine.

FIG. 2 is a schematic diagram ofa preferred embodiment of the circuitry for an energy tester that accomplishes the princples of this invention.

FIG. 3 illustrates the relative magnitude and durations and shapes of the wave forms at various points within the monostable multivibrator illustrated in FIG. 2.

FIG. 4 illustrates the signals being supplied to the comparator.

DETAILED DESCRIPTION OF THE DRAWINGS Referring now to the drawings, FIG. 1 illustrates a block diagram of a blasting machine (1) which is used in combination with the energy testing device embodying the principles of this invention. The energy testing device includes a reference signal generator (3) and a translator or signal generator (2) which supply signals to a comparator (6). The comparator (6) compares the received signals and provides an output signal that visually indicates GO or NO GO". The reference signal generator (3), which is related to an acceptable energy level, includes a trigger circuit (4), which is responsive to the discharge of energy from the blasting machine and which triggers a monostable multivibrator (5) that provides a pulse of fixed magnitude and duration that corresponds to a predetermined amount of energy. The output of the reference signal generator (3) is a pulse type signal, the magnitude of which is substantially constant with respect to time and is represented by the letter A. The signal generator (2), which is related to the amount of energy discharged from the blasting machine, is actuated simultaneously with the trigger cir cuit (4) upon the discharge of energy from the blasting machine (1). The output of the signal generator (2) is a signal that varies in magnitude with respect to time and is represented by the letter B. The comparator (6) receives signal (A) from the reference signal generator (3) and signal (B) from the signal generator (2) and compares them. The comparator (6) provides a signal (C) when the first signal (B) is equal to or exceeds the reference signal (A) at anytime during the presence of the reference signal (A). The comparator (6) also provides a second signal (D) when the reference signal (A) is greater than the first signal (B). Signal (C) actuates an indicating device (GO) that gives audio or visual indication that the blasting machine is putting out the amount of energy that it is supposed to. Signal (D) actuates an indicating device (NO-GO) that gives audio or visual indication that the blasting machine is not putting out sufficient energy.

FIG. 2 is a schematic diagram that illustrates a preferred embodiment of an apparatus that tests the amount of energy discharged from the output of a blasting machine. Portions of the schematic have been outlined with dotted lines which correspond to the functions illustrated in the block diagram shown in FIG. I.

In the schematic the blasting machine (1) is represented by a battery (100), a storage capacitor (101), which is triggered by a gaseous conductor (103), such as a spark gap discharge device, and a high impedance 5 resistor (102). When the gaseous conductor (103) is triggered, the energy stored in capacitor (101) is discharged into the input terminals (8, 9) ofthe testing apparatus. The input terminals (8, 9) of the testing apparatus communicate with the reference signal generator (3) and the signal generator (2).

The signal generator (2), which is outlined by a broken line (elements 201214), includes load resistors (201, 202 steering diode (203), current limiting resistors (205, 206), storage capacitors or energy level detecting capacitors (209, 210), and a resistor (207) to NULL the loading effects of leakage from the storage capacitors (209, 210). Zener diode (211) is used to protect a logic gate (601) which is the input of the comparator (6) from voltage transients. The two resistors (201, 202) are used to simulate different loads, i.e. blasting caps connected in series or in parallel. Switches (214, 204, 312 and 514) are two position switches. When the switches are in position A, the testing apparatus tests a blasting machine for the application of energy to series connected blasting caps. When the switches are in position B, the testing apparatus tests a blasting machine for the application of energy to parallel connected blasting caps or squibbs. The values of resistors (205, 206) and capacitors (209, 210), with their respective load resistors (201, 202) are chosen such that if the blasting machine capacitor (101) voltage and the trigger gap voltage (103) varies over normal tolerances for the same stored energy (e.g. 26 Joules), the voltage across capacitors (209) or (210) change less than 50 millivolts. The values of the resistors (201, 202, 205, 206) and capacitors (209, 210) of the signal generator having been chosen so that the voltage across capacitors (209) or (210) will attain the threshold level of the comparator gate (601) (8.0 volts) when 26 Joules of energy are discharged through the signal generator when the switches (214, 204, 212 and 514) are in position A or B. A change of 1 Joule or more in the energy discharged into the tester from the blasting machine will change the voltage generated across capacitor (209) or (210). It has been determined by design that for every one Joule change in energy discharged into the tester, the voltage change across capacitors (209) or (210) will change 100 millivolts. Obviously, the input gate (601) of the comparator (6) is sensitive enough to detect changes of 100 millivolts.

The reference signal generator (3), which is outlined by a broken line, comprises a trigger circuit (elements 401-413) and a monostable multivibrator (elements 501-511). The trigger circuit (4), like the reference signal generator (2), is directly connected to the input (8, 9) of the testing apparatus. It is the function of the trigger circuit (4) to provide a trigger spike to the monostable multivibrator (5). The trigger circuit (4) uses a Darlington-type amplifier (transistors 406, 407) for an input stage which, with resistors (401, 404), has a very high input impedance, so that the energy discharged from the blasting machine into the tester is dissipated mostly in the reference signal generator (2). Therefore, the energy required to operate the trigger circuit (4) is negligible and substantially all the output energy of the blasting machine is directed to the reference signal generator (2). The output of the Darlington amplifier (transistors 406, 407) is differentiated by capacitor (408) and resistors (410) to turn the output transistor (412) ON causing a positive pulse (Tl, FIG. 3) at the collector of transistor (412). This positive output pulse is used to trigger the monostable multivibrator (5) ON" when it is transmitted through lead (51) to the input gate (501) of the monostable multivibrator (5). The primary purpose of the trigger circuit (4) is to provide an acceptable wave shape (T1) to the input of the monostable multivibrator (5).

The monostable multivibrator (5) includes elements (501-514) and test jack (50). The duration of the output pulse (T2) of the monostable multivibrator (5) is independent of the duration of the input pulse (T1) re ceived from the trigger circuit (4) through lead (51). The magnitude and duration of the output pulses (T2) are determined by capacitors (505) or (506) and the internal resistance of gate (503). To obtain a long output pulse, the capacitance of capacitors (505) and (506) is adjusted to be in the order of 0.6 microfarads or less. Switch (514) serves the same purpose as switch (214) of the signal generator (2). Gates (501, 502, 503 and 507) are diode transistor logic elements with two inputs and four gates. When transistor (509) is OFF, capacitors (505) and (506) are charged through resistor (508). When transistor (509) is turned ON, capacitor (505) and (506) discharge. Transistor (509) is turned OFF very quickly when the leading edge of the trigger pulse (T1) occurs at input (2) of gate (501 When the leading edge of the input signal pulse (Tl) turns the transistor (509) OFF," the base voltage of the transistor goes to minus (V in minus Vbe) which is minus 14.3 volts. The capacitors (505) or (506) then start to charge through resistor (508) and when the base to emitter voltage of the transistor (509) (0.7 volts) is reached, transistor (509) is turned ON," concluding the output pulse (T2). In this circuit, the duration of the output pulse (T2) may be computed as 0.7 RC where R is resistor (508) and C is capacitor (505) or (506). The operational characteristics of the monostable multivibrator (5) may be seen by referring to FIG. 3, explained later.

The voltage comparator (6), outlined by a broken line, (elements 601 through 609) includes gates (601, 602, 603 and 604) which are diode transistor logic circuits with two inputs and four gates. Capacitor (605) is designed to eliminate transient spikes in the circuit caused by the discharge of the blasting machine into the testing apparatus. The comparator (6) is designed to provide an output signal (C) from gate (603) if the magnitude of the signal (B) applied to input (62) of gate (601) exceeds the magnitude of signal (A) at input (61) at anytime during pulse (T2) and to provide an output signal (D) from gate (604) if the magnitude of the signal (B) applied to input (62) of gate (601) is less than the magnitude of signal (A) at input (61) at anytime during the presence of pulse (T2) at input (61) of gate (601). An output signal (D) from gate (604) illuminates the NO-GO indicator (714) while an output signal (C) from gate (603) illuminates the GO indicator (728).

The indicator circuit (7) (elements 709-729), which is shown within a broken line, is a simple switching circuit using transistors (711, 722, 727) and resistors (709, 712, 720, 713, 721, 729, 723) in a switching arrangement to turn on indicator lights (714, 728) to give GO and NO-GO signals in response to the output of the comparator (6).

FIG. 3 is a timing diagram that illustrates the wave forms at particular points (A, B, D, E and F) throughout the monostable multivibrator (5). The output of the monostable multivibrator (5) from gate (502) is transmitted through lead (61) to the input (61) of gate (601) of the comparator (6). The timing diagram illustrates that the output pulse (T2) is initiated upon receipt of input pulse (T1) and terminates when transistor (509) turns ON. See wave form F, which illustrates how the base of the transistor (509) goes from a negative potential to a positive potential to turn transistor (509) ON by the charging action of capacitors (505 and 506). The duration (T2) and magnitude (M) of the pulse formed by the monostable multivibrator (5) may be adjusted to any predetermined duration and magnitude. The magnitude of the pulse being representative of a selected energy level.

FIG. 4 illustrates the operational characteristics of comparator (6). Wave form A illustrates the pulse generated by the reference signal generator which is the output of the multivibrator (5) applied through lead (61) to the input of gate (601). Curves B and B illustrate the wave shapes introduced through lead (62) to the input of gate (601) and are representative of the voltage generated across capacitor (209) or (210). When the magnitude of the signal (B) applied to input (62) of gate (601) exceeds (Curve B) the magnitude of the reference signal (A) applied to the input (61) of gate (601) for the duration (T2), the comparator provides a signal (C) that turns GO" lamp (728) ON. However, when the magnitude of signal (B) applied to input (62) of gate (601) does not exceed (Curve B) the magnitude of the reference signal (A) applied to lead (61) of gate (602) at anytime during the duration (T2) of the signal (A), a signal is generated by the comparator that turns NO-GO" indicator (714) ON. In this sense, the comparator (6) may be considered a threshold detector since it supplies an output signal when a predetermined threshold voltage is exceeded.

OPERATION Referring now to the FIGURES, the apparatus for testing the acceptability of the output energy level of a blasting machine, operates as follows: The capacitor (101) of the blasting machine (1) is charged to its full value by the battery and connected to the inputs (8 and 9) of the tester. Trigger tube (103) is triggered to allow the energy in capacitor (101) to discharge into the testing apparatus. Assuming that the load to be tested is a plurality of series connected blasting caps, the switches (214, 204, 212 and 514) are placed in position A. Upon discharge of the energy from the storage capacitor (101), trigger circuit (4) is triggered, supplying a shaped wave form to monostable multivibrator (5) which in turn applies a reference signal (A) to the input (61) of gate (601) of comparator (6). The high input impedance of the trigger circuit (4) causes most of the energy from the storage capacitor (101) to be dissipated in the signal generator circuit (2). Simultaneously with the triggering of the trigger circuit (4), the signal generator (2) receives most of the energy from capacitor (101) and begins to charge capacitor (209) which in turn applies a signal (B), the magnitude of which varies with respect to time, to the other input (62) of the gate (601) of the comparator (6). The magnitude and duration of signal (A) supplied to input (61) is related to a predetermined amount of energy and when signal (A) is exceeded by a signal (B) generated by the signal generator (2) it is known that the energy discharged by capacitor (101) exceeded that predetermined value. The signals from the monostable multivibrator (5) and the signal generator (2) are calibrated by supplying known amount of energies to the circuit. Upon receipt of the two signals (A and B) through input signals (61, 62) the comparator (6) supplies sig nals to the indicator circuitry (7). Referring now to FIGS. 1 and 4, it can be seen that the comparator (6) applies a signal (C) to the indicator circuitry (7) to illuminate the GO" indicator (728) when the magnitude of signal (A) from the reference generator drops below the magnitude of signal (B) from the signal generator (Curve B, FIG. 4). Conversely, the comparator (6) provides a signal (D) to the NO-GO" indicator (714) of the indicator circuitry (7) when the magnitude of the reference signal (A) from the monostable multivibrator (5) remains greater than the magnitude of signal (B) from the signal generator (2) (Curve B, FIG. 4). Referring briefly to FIG. 4, it is to be noted that Curve B, which exceeds the magnitude of signal (A) during the time T2, causes the comparator (6) to generate a signal (C) which gives a GO" indication, and further that Curve B' is a signal, the magnitude of which does not exceed the magnitude of signal (A) during the time period T2 and therefore the comparator (6) generates a signal (D), giving a NO-GO" indication.

While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art, that changes may be made to the invention as set forth in the appended claims and, in some instances, certain features of the invention may be used to advantage without corresponding use of other features. For example, it is to be noted that a signal could be the presence or absence of a voltage depending on the system approach used. Further, the tester and appropriate switching circuitry could be permanently connected to the blasting machine circuitry to render the blasting machine inoperable if it were not operating properly. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having described the invention, what is claimed is: 1. Apparatus comprising a blasting machine of the type that discharges energy stored in a capacitor to initiate a plurality of electro-explosive devices, means for indicating when said energy discharged from said blasting machine is above a predetermined energy value, said means comprising:

circuit means including a series connected resistor and capacitor for receiving said energy discharged from said blasting machine and for producing a first signal which is related to the amount of energy discharged; means for generating a reference signal which is related to a predetermined amount of energy;

means for comparing said reference signal to said first signal and generating a third signal when said amount of energy discharged from said blasting machine is not less than said predetermined amount of energy; and

means responsive to said third signal for indicating the presence of said third signal, whereby said indicating means indicates that the energy discharged from said blasting machine is above said predetermined amount.

2. The combination as recited in claim 1, wherein said means for generating a reference signal which is related to a predetermined amount of energy comprises a trigger circuit responsive to the receipt of an output energy pulse from said blasting machine, said trigger circuit having an input resistance greater than 1 million ohms and means for generating a trigger pulse upon receipt of said blasting machine output energy pulse; and a monostable multivibrator, having input means for receiving said trigger circuit pulse and output means for producing a pulse having a fixed magnitude in response to said trigger circuit pulse; and wherein said first signal increases in magnitude with respect to time.

3. The combination as recited in claim 1 wherein said energy receiving means includes means for dissipating substantially all of said energy received from said blasting machine.

4. The combination as recited in claim 2 wherein said energy receiving means includes means for dissipating substantially all ofsaid energy received from said blasting machine.

5. The combination as recited in claim 3 wherein said energy dissipating means includes a resistor having a resistance at least three decades less than the input resistance of said trigger circuit.

6. The combination as recited in claim 4 wherein said energy dissipating means includes a resistor having a resistance at least three decades less than the input re sistance of said trigger circuit.

7. The combination as recited in claim 1 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.

8. The combination as recited in claim 2 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.

9. The combination as recited in claim 3 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.

10. The combination as recited in claim 4 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.

11. The combination as recited in claim 5 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.

12. The combination as recited in claim 7 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.

13. The combination as recited in claim 8 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.

14. The combination as recited in claim 9 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.

15. The combination as recited in claim 10 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.

16. The combination as recited in claim 11 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal. 

1. Apparatus comprising a blasting machine of the type that discharges energy stored in a capacitor to initiate a plurality of electro-explosive devices, means for indicating when said energy discharged from said blasting machine is above a predetermined energy value, said means comprising: circuit means including a series connected resistor and capacitor for receiving said energy discharged from said blasting machine and for producing a first signal which is related to the amount of energy discharged; means for generating a reference signal which is related to a predetermined amount of energy; means for comparing said reference signal to said first signal and generating a third signal when said amount of energy discharged from said blasting machine is not less than said predetermined amount of energy; and means responsive to said third signal for indicating the presence of said third signal, whereby said indicating means indicates that the energy discharged from said blasting machine is above said predetermined amount.
 2. The combination as recited in claim 1, wherein said means for generating a reference signal which is related to a predetermined amount of energy comprises a trigger circuit responsive to the receipt of an output energy pulse from said blasting machine, said trigger circuit having an input resistance greater than 1 million ohms and means for generating a trigger pulse upon receipt of said blasting machine output energy pulse; and a monostable multivibrator, having input means for receiving said trigger circuit pulse and output means for producing a pulse having a fixed magnitude in response to said trigger circuit pulse; and wherein said first signal increases in magnitude with respect to time.
 3. The combination as recited in claim 1 wherein said energy receiving means includes means for dissipating substantially all of said energy received from said blasting machine.
 4. The combination as recited in claim 2 wherein said energy receiving means includes means for dissipating substantially all of said energy received from said blasting machine.
 5. The combination as recited in claim 3 wherein said energy dissipating means includes a resistor having a resistance at least three decades less than the input resistance of said trigger circuit.
 6. The combination as recited in claim 4 wherein said energy dissipating means includes a resistor having a resistance at least three decades less than the input resistance of said trigger circuit.
 7. The combination as recited in claim 1 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.
 8. The combination as recited in claim 2 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.
 9. The combination as recited in claim 3 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.
 10. The combination as recited in claim 4 wherein said indicator means includes a first light that illuminates for a predetermined period of time in the absence of said third signal.
 11. The combination as recited in claim 5 wherein said indicator means includes a first light that iLluminates for a predetermined period of time in the absence of said third signal.
 12. The combination as recited in claim 7 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.
 13. The combination as recited in claim 8 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.
 14. The combination as recited in claim 9 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.
 15. The combination as recited in claim 10 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal.
 16. The combination as recited in claim 11 wherein said indicator means includes a second light that illuminates for a predetermined period of time in the presence of said third signal. 